My best guesss would be that something like violence or a beheading could be politcially relevant and spark conversation. Whereas nudity is mostly just there to look at and there's porn sites for that.
The people of FEMEN would like a word.
And breast feeders.
And every damn baby picture ever (oh yes, pictures of 2 year olds count as "nudity" for FB)
And...
Well, no, I don't want to go on. I can understand not allowing porn, perhaps even (though that's a cultural thing) full frontal nudity. Breasts? Most of the people have'm, guys. Gunshot wounds, chopped off arms, headshotted babies, decapîtated dogs,...I've seen plenty of graphically violent crap I *really* wouldn't want a 5 year old to see on public pages of FB, and not get taken down.
Again, it's the famous American puritanism, along with considering violence normal and weapons something to worship. You know we all think your culture's a bit messed up, right?